In order to increase the adherence of zinciferous metal-plated steel sheet to paint or other types of dryable films coated thereon, treatment of zinciferous metal-plated steel sheet with an acidic aqueous solution (particularly aqueous phosphate solutions) is widely used in industry in order to form a paint undercoat film on the sheet. Paint films laid down on such a phosphate film layer perform well when the treatment has been managed so as to give appropriate film weights and crystal dimensions. Since these physical parameters must be adjusted into appropriate ranges, it therefore becomes necessary to vary the conversion treatment conditions as a function of the type of plating on the steel sheet and the steel sheet line speed and to frequently remove the sludge that is generated in the treatment bath.
In the case of low-lead hot-dip galvanized steel sheet (with a lead content in the zinc plating layer reduced from that in conventional hot-dip galvanized steel sheet), it has already been discovered that corrosion inhibition is obtained due to the absence of lead segregation at the grain boundaries of the crystals in the plating layer and at the interface between the plating layer and alloy layer. Low-lead hot-dip galvanized steel sheet is, as a consequence, widely used in various industrial sectors. However, unlike the conventional hot-dip galvanized steel sheet, low-lead hot-dip galvanized steel sheet resists cracking during bending processes, with the result that shear stresses become concentrated in the phosphate film positioned between the steel sheet and paint film. This produces cohesive failure in the phosphate film, which in turn causes facile delamination of the paint film.
In order to avoid the problems described above, application-type chromate treatments that include hexavalent chromium and trivalent chromium are in use as paint undercoat treatments in place of phosphate treatments. The treatment bath composition in this type of process is easily maintained and managed. Moreover, this type of process can easily respond to many different types of plating and to line speed variations, and the treatment effluent in this case poses few environmental problems. However, the paint adherence of these chromate films is not as good as that of the phosphate films, and in particular delamination of the paint film occurs quite easily during strong flexural working involving pressure contact.
In order to improve the paint adherence of such application-type chromate films, i.e., in order remediate the problem of facile delamination, (1) Japanese Patent Publication Number Sho 43-12974 [12,974/1968], (2) Japanese Patent Publication Number Sho 52-22618 [22,618/1977], (3) Japanese Patent Publication Number Sho 52-43171 [43,171/1977], and (4) Japanese Patent Application Laid Open [Kokai or Unexamined] Number Sho 61-69978 [69,978/1986]propose methods for improving the paint adherence by preliminarily subjecting zinciferous metal-plated steel sheet to heavy metal substitutional plating with Ni, Co, and/or Fe, and so forth, prior to the execution thereon of the chromating treatment.
Considering these previous methods, (1) Japanese Patent Publication Number Sho 43-12974 relates to a method in which zinciferous metal-plated steel sheet is treated with a basic aqueous solution (pH.gtoreq.11) containing Co.sup.2+, Fe.sup.2+, Fe.sup.3+, or Ni.sup.2+ prior to execution of a chromate treatment on the sheet. However, large amounts of sludge are produced in this method due to the accumulation of the zinc ion eluting into the treatment bath with elapsed treatment time. This necessitates a sludge removal step, which impairs the workability.
(2) Japanese Patent Publication Number Sho 52-22618 and (3) Japanese Patent Publication Number Sho 52-43171 relate to methods in which galvanized steel sheet is treated with an acidic solution (pH around 1.5) that contains Ni.sup.2+, Co.sup.2+, Fe.sup.2+, and/or Fe.sup.3+ prior to execution of a chromate treatment on the sheet. For the purpose of regulating the pH, the acidic substitutional plating baths disclosed in (2) Japanese Patent Publication Number Sho 52-22618 and (3) Japanese Patent Publication Number Sho 52-43171 contain an inorganic acid such as hydrochloric acid, sulfuric acid, hydrofluoric acid, or fluosilicic acid, or an organic acid such as citric acid, acetic acid, oxalic acid, and so forth. The zinc ion eluted into the plating bath is present in dissolved form through the formation of a salt with the inorganic acids or through complex formation with the organic acids. The pH in substitutional plating baths of this type is readily increased by the increase in zinc ion concentration, which results in a decline in the substitutional plating reactions. Due to this, large quantities of inorganic acid must be added in order to maintain the pH of the plating bath at the desired values, and this facilitates corrosion of, for example, stainless steel plating bath tanks, pipes, and so forth. Therefore such baths are quite difficult to implement on a practical basis without the use of a stainless steel reactor and piping both coated with rubber lining or the like. This use of corrosive acid, particularly a volatile one such as hydrochloric acid, also causes a deterioration in the working environment.
Finally, (4) Japanese Patent Application Laid Open Number Sho 61-69978 concerns a method in with low-lead hot-dip galvanized steel sheet is treated with an aqueous alkaline solution that contains Fe, Co, and/or Ni or with an aqueous hydrochloric acid solution, aqueous sulfuric acid solution, or aqueous phosphoric acid solution that contains Fe, Co, and/or Ni. In the case of the aqueous hydrochloric acid solution, aqueous sulfuric acid solution, and aqueous alkaline solution, deposition of these metals is impaired by the increase in Zn ion and increase in pH that occur with elapsed treatment time. On the other hand, in the case of phosphoric acid, it would appear that its pH buffering capacity suppresses the increase in pH, and that substitutional deposition of the Fe, Ni, and/or Co then proceeds smoothly. However, absolutely no explanation of this point can be found in the specification of document (4).
At the same time, chromate treatments have also been applied to zinciferous metal-plated steel sheet for the purpose of improving the corrosion resistance. While this type of treatment very effectively inhibits the development of white rust, black rust (also known as blackening) still occurs during storage and transport. A countermeasure to this problem of post-chromating black rust consists, for example, of flash treatment by Ni, Co, or Fe as disclosed in Japanese Patent Publication Number Hei 3-49982 [49,982/1991].
In the technology described in Japanese Patent Publication Number Hei 3-49982, black rust inhibition is achieved by the pre-chromating treatment of zinciferous metal-plated steel sheet with a treatment bath that has a pH of 1 to 4 or 11 to 13.5 and that contains Ni.sup.2+ ion or Co.sup.2+ ion. Even this method, however, suffers from a reduced workability due to the production of sludge that occurs when the zinc ion concentration becomes elevated during the course of treatment.